This invention relates to the preparation of polymers, and more particularly to their preparation by solid state polymerization.
Solid state polymerization (SSP) as a method for preparing polycarbonates is disclosed, for example, in U.S. Pat. Nos. 4,948,871, 5,204,377 and 5,717,056, the disclosures of which are incorporated herein. It is typically described as involving a first step of forming, typically by melt polymerization (i.e., transesterification) of a dihydroxyaromatic compound such as 2,2-bis(4-hydroxyphenyl)propane (bisphenol A) with a diaryl carbonate such as diphenyl carbonate, a precursor polycarbonate which is often (though not necessarily) an oligomer; a second step of enhancing the crystallinity of the precursor polycarbonate; and a third step of building molecular weight by heating the enhanced crystallinity precursor polycarbonate to a temperature between its glass transition temperature and its melting temperature. Use of this method is of increasing interest by reason of its effectiveness and environmental benefits. A similar method is used for polyesters, especially bottle grade poly(ethylene terephthalate), for which an oligomer is prepared by the reaction of a dicarboxylic acid ester with a diol and is then crystallized and subjected to SSP to build molecular weight.
The crystallinity enhancement step is considered essential in accordance with the aforementioned prior art. As taught, for example, in U.S. Pat. No. 4,948,871, the crystallinity of the precursor polycarbonate should be in the range of about 5-55% as determined, for example, from powder X-ray diffraction patterns. If it is below 5%, the melting point of the precursor polycarbonate is so low that melting rather than SSP may occur. On the other hand, at crystallinity levels greater than 55% the polymerization rate is too low to be practical.
Crystallinity enhancement may be performed by several methods. These include heat treatment, solvent treatment, action of crystallization promoters and treatment with swelling agents. Each of these methods requires time input and/or treatment with extraneous chemicals which must be kept in inventory and stored. It would be desirable, therefore, to develop an SSP method which does not require the crystallinity enhancement step.